The present invention relates to hydrostatic transmission (xe2x80x9cHSTxe2x80x9d) systems and controls therefore, and more particularly, to closed loop hydrostatic transmission systems of the type including a charge pump which serves as the source of make-up fluid to the closed loop, and as the source of control fluid to the system controls.
However, it should be understood that the present invention may also be utilized advantageously in HST systems in which the pump is controlled by means other that hydraulically, by control pressure from a charge pump. For example, the invention would provide substantial benefit when used with a pump in which the angle of the swashplate is controlled manually by means of a mechanical handle. In a pump of this type, and as is well known to those skilled in the art, the pump still includes a charge pump, but the sole function of the charge pump on a mechanically actuated pump is to provide make-up fluid to the closed loop, to compensate for leakage.
A typical closed loop hydrostatic transmission system of the type to which the present invention relates is illustrated and described in U.S. Pat. No. 4,936,095, assigned to the assignee of the present invention and incorporated herein by reference. In such closed loop HST systems, the closed loop and the various system components are protected from excessive pressures by means of a pair of high pressure (xe2x80x9coverpressurexe2x80x9d) relief valves, which are typically included in a valve assembly disposed in the pump end cover.
In addition, the typical closed loop HST system includes a bypass valve assembly which is capable of xe2x80x9ccross portingxe2x80x9d (or short-circuiting) the closed loop hydrostatic circuit. One of the primary uses of the bypass valve assembly arises whenever the vehicle is disabled and must be moved (such as by being towed) a short distance. The bypass valve assembly usually functions by means of some sort of mechanical device which may be actuated to push (lift) both of the check valves off of their respective check valve seats, thus allowing oil to flow freely from one side of the hydrostatic closed loop to the other side. In the condition described, the motor is then able to turn freely as the vehicle is being towed, without causing the fluid to overheat.
Although the above-described prior art arrangement for short-circuiting the hydrostatic closed loop has been functionally acceptable, the need to add some sort of mechanical structure to the bypass valve assembly, which is able to unseat both of the check valves, does add substantially to the complexity and cost of the bypass valve assembly, and of the overall HST system. Also, it is quite common that the pump of the HST system is xe2x80x9cburiedxe2x80x9d deep within the vehicle, such that the vehicle operator does not really have access to the bypass valve assembly (also referred to sometimes as the xe2x80x9ctow valvexe2x80x9d). For such a vehicle, in the event that the vehicle needs to be towed, there may have to be some disassembly of the vehicle, in order for the operator to have access to the mechanical device for cross-porting the bypass valve assembly. Such partial disassembly would add substantially to the overall cost and inconvenience of the vehicle down-time.
In addition, those skilled in the art of closed loop HST control systems have become aware of various other operating situations in which it would be desirable to short circuit the hydrostatic closed loop, but wherein actuation of the mechanical device for unseating the check valves would not be feasible. For example, whenever the vehicle operator applies the vehicle brakes, it would be desirable to ensure that the motor is not being positively driven by pressurized fluid from the pump. It has been common practice on vehicles propelled by HST systems to provide a parking brake having a xe2x80x9cload holdingxe2x80x9d torque greater than the torque output of the HST system, just to make sure that the vehicle can""t xe2x80x9cdrive throughxe2x80x9d the vehicle parking brake. Such a large capacity parking brake adds to the overall cost of the vehicle, without any added benefit.
On servo pumps (i.e., those on which the pump displacement is controlled hydraulically), one of the solutions used to prevent driving through the parking brake is to add a special valve which, whenever the vehicle parking brake is applied, will be actuated to block communication of control pressure from the charge pump to the servo control device. This solution is generally functionally acceptable, but adds to the cost and complexity of the pump and the controls, especially because there would now be the need for inter-connection between the parking brakes and the pump controls, for no purpose other than to prevent the xe2x80x9cdriving-through-the-brakesxe2x80x9d problem.
Another situation in which it is desirable to short-circuit the hydrostatic closed loop, and an example of a situation which is an issue of safety, is the situation in which the vehicle operator is not present in the operator seat (as is typically determined by some sort of electrical seat sensor). On many vehicles, an electrical signal indicating the absence of the vehicle operator from the operator""s seat is used to take some safety-related action, such as not permitting operation of the engine ignition system, or in the case of an HST having electronic pump controls, not permitting the pump swashplate to move from its normal, zero-swash position to a displaced position. Prior to the present invention, there would have been a need for some arrangement to destroke the pump, in response to the safety issue, in addition to whatever structure was required to deal with the parking brake issue discussed previously.
Accordingly, it is an object of the present invention to provide an improved closed loop hydrostatic transmission and control system which overcomes the disadvantages of the prior art mechanical arrangement for unseating the check valves to short-circuit the hydrostatic closed loop.
It is another object of the present invention to provide an improved closed loop hydrostatic transmission and control system in which it is possible to achieve the desired short-circuiting of the hydrostatic closed loop in response to an electrical signal.
It is still another object of the present invention to provide an improved method of controlling a closed loop hydrostatic transmission system in which the hydrostatic closed loop may be short-circuited in response to an electrical input signal generated as the result of the occurrence of any one or more predetermined vehicle conditions.
It is yet another object of the present invention to provide an improved method of controlling a closed loop hydrostatic transmission system, which accomplishes the above-stated objects, and which, on many vehicles, makes it possible to utilize a lower torque capacity parking brake.
The above and other objects of the invention are accomplished by the provision of an improved closed loop hydrostatic transmission system including a pump adapted to supply pressurized fluid, a motor adapted to receive the pressurized fluid, and high pressure and low pressure conduits for communicating fluid between the pump and the motor. A source of relatively low pressure make-up fluid is adapted for fluid communication with the low pressure conduit. A relief and check valve assembly is disposed between the source of make-up fluid and the high pressure conduit. The assembly includes a relief valve member operable, in the presence of fluid pressure in the high pressure conduit in excess of predetermined maximum pressure to be biased open in opposition to the biasing force of a relief spring to permit fluid communication from the high pressure conduit to the low pressure conduit. The assembly includes a check valve member operable, in the presence of fluid pressure in the high pressure conduit which is less than the pressure of the relatively low pressure make-up fluid, to be biased open, in opposition to the biasing force of a check spring, to permit fluid communication from the source of make-up fluid to the high pressure conduit.
The improved closed loop hydrostatic transmission is characterized by the check spring being disposed in a spring chamber bounded on one side by a member which is substantially fixed relative to the check valve member in the absence of relatively high pressure in the high pressure conduit. A normally closed drain valve has an inlet in fluid communication with the spring chamber, and an outlet in fluid communication with the system reservoir. In response to a predetermined input, the drain valve will move from its normally closed condition to an open condition, draining the spring chamber to the system reservoir, and causing the check valve member to open under the influence of the relatively low pressure make-up fluid.
In accordance with another aspect of the invention, an improved method of controlling a closed loop hydrostatic transmission is provided wherein the transmission is as described previously. The relief and check valve assembly is disposed between the source of make-up fluid and the high pressure conduit and includes a first check valve member operable in the presence of fluid pressure in the high pressure conduit which is less than the pressure of the make-up fluid, to be biased open in opposition to the force of a first check spring. The assembly further includes a second check valve member operable, in the presence of fluid pressure in the low pressure conduit which is less than the pressure of the make-up fluid, to be biased open in opposition to the biasing force of a second check spring. The method of controlling comprises the step of sensing the existence of a predetermined vehicle condition and generating an electrical signal representative of the condition.
The improved method of controlling a closed loop hydrostatic transmission is characterized by disposing the first and second check springs in first and second spring chambers, respectively, the chambers being bounded on one side by first and second members, respectively, which are substantially fixed relative to the first and second check valve members, respectively, in the absence of relatively high pressure in the high pressure and low pressure conduits. The method includes providing a normally closed electromagnetically actuated drain valve having an inlet in fluid communication with both of the first and second spring chambers, and an outlet in fluid communication with the system reservoir. Finally, the method includes transmitting to the drain valve the electrical signal to move the drain valve from its normally closed condition to an open condition, draining the spring chamber to the reservoir and opening both of the first and second check valve members, thereby permitting relatively unrestricted fluid communication between the high pressure and low pressure conduits.